It's not just about using 2s large buffers. If you use large buffers like that you also need to allow rewriting of them (which means "rewinding" of all filters in the system and so on). Because if you couldn't rewrite them pressing stop or start in your media playback would actually only result in what you hear 2s later.
Allowing rewrite of audio buffers is what JACK doesn't do. And adding this makes things very complicated. I did it for PA, but I simply see no reason for JACK to do it as well. Also, since JACK allows much more complex routing than PA, the rewriting buffer issue becomes equally more complex. i.e. every part that becomes part of a JACK pipeline would need to be able to "rewind" its state. That means every JACK client application would have to be patched non-trivially.